THE MISSISSIPPI VALLEY. 15 



Cumberland river is the next most important arm of the Ohio, which it 

 enters only 16 miles above the mouth of the Tennessee. The elevation of low 

 water is here about 284 feet above tide. 



The surface of the Cumberland, at Nashville, is 388* feet above the Atlantic, 

 or 104 feet above the mouth of the river. The distance by the river, from its 

 entrance into the Ohio, to Nashville, is 240 miles, and the average fall, there- 

 fore, 6^ inches per mile — very nearly the same as that of the corresponding 

 portion of its sister stream, the Tennessee, but about twice as great as that of 

 the Ohio below the falls, and that of the Mississippi below the mouth of the 

 Ohio. 



The Wabash, next in succession, but perhaps equal in volume to the Cum- 

 berland, is the largest of the tributaries of the Ohio which descend along its 

 northern plane. The elevation of low water at the mouth of the Wabash is 297 

 feet above tide. In the first 91 miles, extending from its confluence with the 

 Ohio to the mouth of White river, the fall is 57 feet, or 7^ inches per mile.f 



The foot of the "Grand rapids" is one mile above the mouth of White river. 

 These ripples have an aggregate fall of 10 feet, distributed over a space of eight 

 miles. 



The inclination of this great river from point to point is exhibited on the 

 profile. The total descent from the mouth of Little river to the Ohio, a 

 distance computed at 370 miles, is 385 feet, or a small fraction over 12 inches 

 per mile. J 



It is worthy of observation that the rate of descent of the tributaries of the 

 Ohio increases very nearly in proportion to the increase of the slope of the 

 Ohio itself, as we ascend from its mouth to its source. Those rivers which 

 enter below the Falls have gentler currents than those which enter above the 

 Falls ; and those which enter above the Falls are more languid than those which 

 come in near its head. 



The rate of descent of all these navigable tributaries, near the Ohio, is about 

 twice as great as that of the Ohio itself where joined by the respective tributa- 

 ries. Thus, the Tennessee, and Cumberland, and Wabash, have each a descent 

 of about 7 inches per mile, while the average slope of the Ohio below the falls 

 is 3 inches per mile. We shall find other examples of this rule as we ascend ; 

 but the next river in order is an exception which justifies explanation. 



Green river enters on the left border of the Ohio, from the State of Ken- 

 tucky. The average inclination of this stream, from Bowling Green, on Barren 

 river, a tributary of Green river, to its mouth — a distance of 175 miles — is 4^ 

 inches per mile. The actual fall in this distance is 60 feet, and the rate of incli- 

 nation but one-third greater than that of the lower Ohio. 



To comprehend the structure of the country and the cause of this exception, 



* From surveys of J. Edgar Thomson and James H. Grant, civil engineers. 



t Letter of Sylvanus Lothrop. 



t Report of J. L. Williams and Howard Stansbury, civil engineers. 



